[gomp4] Add tables generation

[Ilya Verbin]

On 18 Aug 18:24, Bernd Schmidt wrote:
> I think I'd be happier if the function was called init_offload_table
> and also collected functions, rather than leaving that to
> expand_omp_target. I think the patch would be ok with that change.

For the functions it's not so easy to identify which of them to add into the table, e.g.:
  #pragma omp target
    #pragma omp parallel
      x++;
Here 2 functions with "omp declare target" attribute are created.  But only the outer must be added to the table.
So I believe that expand_omp_target is better place for the functions.

  -- Ilya